Ionization gauge for high pressure operation

ABSTRACT

An ionization gauge to measure pressure, while controlling the location of deposits resulting from sputtering when operating at high pressure, includes at least one electron source that emits electrons, and an anode that defines an ionization volume. The ionization gauge also includes a collector electrode that collects ions formed by collisions between the electrons and gas molecules and atoms in the ionization volume, to provide a gas pressure output. The electron source can be positioned at an end of the ionization volume, such that the exposure of the electron source to atom flux sputtered off the collector electrode and envelope surface is minimized. Alternatively, the ionization gauge can include a first shade outside of the ionization volume, the first shade being located between the electron source and the collector electrode, and, optionally, a second shade between the envelope and the electron source, such that atoms sputtered off the envelope are inhibited from depositing on the electron source.

RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/US2013/025198, filed Feb. 7, 2013, which claims the benefit ofU.S. Provisional Application No. 61/596,470 filed Feb. 8, 2012.

The entire teachings of the above application are incorporated herein byreference.

BACKGROUND OF THE INVENTION

Ionization gauges, more specifically Bayard-Alpert (BA) ionizationgauges, are the most common non-magnetic means of measuring very lowpressures. The gauges have been widely used worldwide. These gauges weredisclosed in 1952 in U.S. Pat. No. 2,605,431, which is hereinincorporated by reference in its entirety. A typical ionization gaugeincludes an electron source, an anode, and an ion collector electrode.For the BA ionization gauge, the electron source is located outside ofan ionization space or anode volume which is defined by a cylindricalanode screen. The ion collector electrode is disposed within the anodevolume. Electrons travel from the electron source to and through theanode, cycle back and forth through the anode, and are consequentlyretained within, or nearby to, the anode.

In their travel, the electrons collide with molecules and atoms of gasthat constitute the atmosphere whose pressure is desired to be measured.This contact between the electrons and the gas creates ions. The ionsare attracted to the ion collector electrode, which is typicallyconnected to ground. The pressure of the gas within the atmosphere canbe calculated from ion and electron currents by the formula P=(1/S)(I_(ion)/I_(electron)), where S is a coefficient with the units of1/Torr and is characteristic of a particular gauge geometry, electricalparameters, and pressure range.

The operational lifetime of a typical ionization gauge is approximatelyten years when the gauge is operated in benign environments. However,these same gauges and electron sources (cathodes) fail in minutes orhours when operated at too high a pressure or during operation in gastypes that degrade the emission characteristics of the electron source.Sputtering is a problem when operating the ionization gauge at highpressures, such as above 10⁻⁴ Torr. This is a problem at high pressurebecause there is more gas to ionize. This sputtering is caused by highenergy impacts between ions and components of the ionization gauge. Ionswith a high energy may impact a tungsten material that forms a collectorpost of the ionization gauge. These impacts result in atoms beingejected from the collector post and envelope surfaces with significantinternal kinetic energies. Ejected material can travel freely to othersurfaces within the line of sight of the sputtered surfaces, and cancause gauge failure by coating the cathode or by coating of thefeed-through insulators of the gauge, which can result in electricleakages.

Therefore, there is a need for an ionization gauge design that minimizesor eliminates the above mentioned problems.

SUMMARY OF THE INVENTION

Coating of the electron source, such as a hot filament, is facilitatedby the typical arrangement, shown in FIG. 1, of the filament andcollector structures side by side to one another in a parallelarrangement, often with a large surface area of the filament facing thecollector surface. The present ionization gauge minimizes the effects ofself-sputtering on filament emission efficiency by inhibiting thesputtered atoms from finding straight paths from the collector to theelectron source in the gauge.

There is provided an ionization gauge to measure pressure whilecontrolling the location of deposits resulting from sputtering whenoperating at high pressure. The ionization gauge can include an anodestructure comprising a mesh grid defining an ionization volume in whichelectrons impact gas molecules and atoms. The mesh grid can be acylindrical mesh grid. The ends of the grid define the ends of theionization volume. The ionization gauge also includes a hot cathodeelectron source that emits electrons. The electron source can bepositioned at an end of the ionization volume. The ionization gaugefurther includes a collector electrode to collect ions formed bycollisions between the electrons and gas molecules and atoms, to providea gas pressure output. The collector electrode extends along a collectoraxis through the ionization volume, the collector axis extending throughthe ends of the ionization volume. Alternatively, the hot cathodeelectron source can be a ribbon filament positioned side by side to thecollector electrode, the ribbon filament having a flat surface orientedat about 90° with respect to the collector electrode, such that thesurface area of the filament facing the collector is minimized.

In yet another alternative, an ionization gauge can include an anodestructure comprising a mesh grid defining an ionization volume in whichelectrons impact gas molecules and atoms, and an electron source thatemits electrons. The mesh grid can be a cylindrical mesh grid. Theionization gauge also includes a first collector electrode to collections formed by collisions between the electrons and gas molecules andatoms, to provide a gas pressure output. The ionization gauge furtherincludes a first shade outside of the ionization volume, the first shadebeing located between the electron source and the first collectorelectrode. One of the first collector electrode and the electron sourceis located inside the anode structure, and the other of the firstcollector electrode and the electron source is located outside the anodestructure. If the source that emits electrons is located outside theanode structure and the first collector electrode is located inside theanode structure, then the ionization volume can be inside the anodestructure, and, optionally, the ionization gauge can further include asecond collector electrode located inside the anode structure. As anadditional option, the ionization gauge can further include a thirdcollector electrode located outside the anode structure, in between thefirst shade and the anode structure. Alternatively, the source thatemits electrons can be located inside the anode structure and the firstcollector electrode can be located outside the anode structure. Theelectron source can be a hot cathode or a microchannel plate.

The ionization gauge can further include an envelope surrounding theelectron source, the anode, and the first collector electrode.Optionally, a second shade can be located between the envelope and theelectron source, such that atoms sputtered off the envelope areinhibited from depositing on the electron source.

A method of measuring pressure with an ionization gauge includesemitting electrons from a hot cathode electron source positioned at anend of an ionization volume, the electrons colliding with gas moleculesand atoms inside an anode structure comprising a cylindrical mesh gridthat defines the ionization volume. The method further includescollecting ions formed by collisions between the electrons and gasmolecules and atoms on a collector electrode to provide a gas pressureoutput. The collector electrode extends along a collector axis throughthe ionization volume, the collector axis extending through the ends ofthe ionization volume. The pressure can be in a range of between about10⁻¹ Torr and about 10⁻⁴ Torr.

Alternatively, a method of measuring pressure with an ionization gaugeincludes emitting electrons from an electron source, the electronscolliding with gas molecules and atoms inside an anode structurecomprising a mesh grid that defines an ionization volume. The mesh gridcan be a cylindrical mesh grid. The method further includes locating afirst shade outside of the ionization volume between the electron sourceand a first collector electrode, one of the first collector electrodeand the electron source being located inside the anode structure, andthe other of the first collector electrode and the electron source beinglocated outside the anode structure. The method also includes collectingions formed by collisions between the electrons and gas molecules andatoms on the first collector electrode to provide a gas pressure output.If the source that emits electrons is located outside the anodestructure and the first collector electrode is located inside the anodestructure, then, optionally, the method can further include locating asecond collector electrode inside the anode structure. As an additionaloption, the method can further include locating a third collectorelectrode outside the anode structure, in between the first shade andthe anode structure. The method can further include surrounding theelectron source, the anode structure, and the collector electrode(s)with an envelope and locating a second shade between the envelope andthe electron source, such that atoms sputtered off the envelope areinhibited from depositing on the electron source.

The present ionization gauge has many advantages, including minimizingthe exposure of the electron source to atom flux sputtered off thecollector electrode and envelope surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of example embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingembodiments of the present invention.

FIG. 1 is a schematic view of a generalized ionization gauge of theprior art.

FIG. 2 is a schematic view of an ionization gauge of the presentdisclosure having an electron source positioned at an end of theionization volume.

FIG. 3A is a cross-section of the ionization gauge shown in FIG. 2 withan envelope surrounding the ionization volume.

FIG. 3B is an illustration of simulated electron paths within theionization gauge shown in FIG. 3A.

FIG. 4A is a side view of an ionization gauge of the present disclosurehaving a shade between the electron source and the collector electrode.

FIG. 4B is a side view of an ionization gauge of the present disclosurehaving a shade between the electron source and the collector electrode,and having the collector electrode formed of two collector electrodes.

FIG. 5A is a top sectional view of an ionization gauge of the presentdisclosure having a shade between the electron source and the collectorelectrode.

FIG. 5B is a top sectional view of an ionization gauge of the presentdisclosure having a shade between the electron source and the collectorelectrode, and having the collector electrode formed of two collectorelectrodes.

FIG. 5C is a top sectional view of an ionization gauge of the presentdisclosure having a shade between the electron source and the collectorelectrode, and having the collector electrode formed of two collectorelectrodes oriented in a plane parallel to the plane of the electronsource.

FIG. 5D is a side view of an ionization gauge of the present disclosurehaving a double loop electron source.

FIG. 6A is a side view of an ionization gauge of the present disclosurehaving an envelope, a first shade between the electron source and thecollector electrodes, a second shade between the envelope and theelectron source, a second collector electrode inside the anodestructure, and a third collector electrode outside the anode structure.

FIG. 6B is a side view of an ionization gauge of the present disclosurehaving an envelope, a first shade between the electron source and thecollector electrodes, a second shade between the envelope and theelectron source, and a collector electrode outside the anode structure.

FIG. 7 is a top sectional view of an ionization gauge of the presentdisclosure having a first shade between the electron source and thecollector electrode, and a second shade between the envelope and theelectron source.

DETAILED DESCRIPTION OF THE INVENTION

As mentioned above, coating of the electron source 105, such as a hotfilament, is facilitated by the typical arrangement, shown in FIG. 1, ofthe filament 105 and collector 110 structures side by side to oneanother in a parallel arrangement, often with a large surface area ofthe filament 105 facing the collector surface 110.

A description of example embodiments of the invention follows.

In one embodiment, shown in FIG. 2, an ionization gauge 100 of thepresent disclosure has an anode structure comprising a cylindrical wiregrid 120 around posts 112 and 114, defining an ionization volume 121 inwhich electrons impact gas molecules and atoms. Two end grids 111 a and111 b define the ends of the ionization volume 121. A hot cathodeelectron source 105 emits electrons 125, the electron source 105 beingpositioned at an end of the ionization volume 121. A collector electrode110 collects ions formed by collisions between the electrons 125 and gasmolecules and atoms, to provide a gas pressure output. The collectorelectrode 110 extends along a collector axis through the ionizationvolume 120, the collector axis extending through the ends of theionization volume 121. At the other end of ionization volume 121 fromthe hot cathode electron source 105, the collector electrode 110 andsupport posts 112 and 114 protrude through the solid disk 116 that isdisplaced downward from end grid 111 b. The electron source 105 can be,for example, a heated cathode filament, as shown in FIG. 2, or a disccathode thermionic emitter (e.g., Kimball Physics, Inc., Wilton, N.H.).

A cross-section of the upper portion of the ionization gauge 100 thatincludes the electron source 105 is schematically illustrated in FIGS.3A and 3B. The ionization gauge 100 is based on the ionization of gasmolecules and atoms in a measurement chamber 117 by a constant flow ofelectrons. The negatively charged electrons 125 shown in FIG. 3B (onlyshown on the left side in FIG. 3B, although electrons would similarly beon the right side) are emitted at a well-controlled selectable ratefrom, for example, a heated cathode 105, and can be released oraccelerated toward a positively charged anode 120. The electrons 125pass into and through the anode 120 and then cycle back and forththrough the anode 120. The electrons 125 are then retained within theionization volume of the anode 121, in part due to the potential bias onend grids 111 a and 111 b. In this space, the electrons 125 collide withthe gas molecules and atoms to produce positively charged ions beforecolliding with a grid wire 120, or end grids 111 a or 111 b. During lowpressure operation, significant ionization only occurs within the anode120, and thus the volume 121 within the anode 120 is referred to as theionization volume 121. Some ionization may occur outside the ionizationvolume 121, in particular during high pressure operation (e.g., aboveabout 10⁻⁴ Torr), when sufficient ionization may occur for collection.The ions are collected by the ion collector 110. Collector 110 is nearlyat ground potential, which is negative with respect to the positivelycharged anode 120. However, this arrangement is not limiting andcollector 110 may have various potential differences with respect to theanode 120. See application Ser. No. 12/860,050 published as US2011/0062961 A1, which is herein incorporated by reference in itsentirety. At a constant cathode to anode voltage and electron emissioncurrent, the rate that positive ions are formed is related to thedensity of the gas in the gauge 100. Turning back to FIG. 2, this signalfrom the collector electrode 110 is detected by an ammeter 135, which iscalibrated in units of pressure, for all pressure readings.

In the embodiment shown in FIG. 2, sputtered atoms from collector 110,that are typically ejected along paths that are perpendicular to thecollector 110, are unlikely to deposit on filament 105, because filament105 is positioned at an end of the ionization volume 121, out of a lineof sight perpendicular to the collector electrode 110, and no longeralongside collector 110, in contrast to the arrangement shown in FIG. 1.The use of a hot cathode filament as the electron source 105 in thegeometry shown in FIG. 2 enables high pressure operation of theionization gauge 100.

Alternatively, as shown in FIGS. 4A, 4B, 5A, and 5B, a first shade 115can be located between the electron source 105 and the collector 110 toeffectively shield the exposed filament surfaces from the effects ofsputtered collector atoms. Electric fields direct the electrons producedby electron source 105 around the shade 115 and into the ionizationvolume 120. The electron source can, for example, be a hot cathode asshown in FIGS. 4A and 4B, or a microchannel plate. For the microchannelplate, see application Ser. No. 12/808,983 published as US 2011/0234233A1, which is herein incorporated by reference in its entirety. The hotcathode 105 can be a cylindrical filament shown as a single loop inFIGS. 4A and 4B, or the filament can be a double loop, as shown in FIG.5D (where the shade 115 is not shown for clarity), or a ribbon filament,shown as top sectional views in FIGS. 5A, 5B, and 5C, the ribbonfilament 105 having flat surface oriented at about 90° with respect tothe collector electrode 110, such that the surface area of the filament105 facing the collector electrode 110 is minimized. One of thecollector electrode 110 and the electron source 105 can be locatedinside the anode structure 120, and the other of the collector electrode110 and the electron source 105 can be located outside the anodestructure 120. As shown in FIG. 6A, if the source 105 that emitselectrons is located outside the anode structure 120 and the collectorelectrode 110 is located inside the anode structure 120, then theionization gauge 200 can include a first collector electrode 110 a and asecond collector electrode 110 b located inside the anode structure 120,and, optionally, a third collector electrode 210 located outside theanode structure 120, in between the first shade 115 and the anodestructure 120, for high pressure measurements of very short mean freepath ions formed in measurement chamber 117. Alternatively, theionization gauge 200 can include a single collector electrode 110located inside the anode structure 120 and a collector electrode 210located outside the anode structure 120, as shown in FIG. 6B. Here, thecollector electrode 210 is outside the ionization volume defined by theanode structure 120, but, at high pressure, ionization also occursoutside this primary ionization volume.

The embodiment in FIGS. 4A, 4B, 5A, and 5B is shown as a nudeconfiguration of the ionization gauge 100, that is, without asurrounding gauge vessel. It is also envisioned that non-nude typeionization gauges are also possible, having an envelope 205 as shown inFIGS. 6A, 6B, discussed above and FIG. 7, that shows a top sectionalview of the ionization gauge shown in FIG. 6B. At pressures greater thanabout 10⁻⁴ Torr, electrons emitted from the filament 105 have a highprobability of colliding with gas atoms or molecules on the way to theanode 120.

As ions are formed outside the anode 120, they are accelerated towardsthe envelope 205, typically made of stainless steel, and sputtering ofstainless steel is now possible. Some of the component atoms ofstainless steel sputtered from the envelope 205 coat the back surface ofthe filament 105. As the pressure continues to increase and approachesabout 10⁻¹ Torr, the majority of ionization occurs outside the anode 120and stainless steel wall sputtering becomes the main source of materialdeposited on the filament 105. The resulting typical deposition patternon the filament 105 is a coating of stainless steel component atoms onthe side of the filament 105 facing the envelope 205 and a coating oftungsten on the side of the filament 105 facing the collector 110. FIGS.6A, 6B, and 7 show specific non-nude type ionization gauges 200, whereina second shade 119 is used to shield the electron source 105 from theeffects of atoms sputtered off the envelope 205, that is, atomssputtered off the envelope 205 are inhibited from depositing on theelectron source 105. The first and second shades, 115 and 119,respectively, can be shaped metal plates, such as, for example,stainless steel. The electric potential of the shades can be the same asthe cathode potential or slightly lower than the cathode potential, andtherefore not subject to the sputtering problems discussed above.

A method of measuring pressure with an ionization gauge 100 describedabove and shown in FIG. 2 includes emitting electrons from a hot cathodeelectron source 105 positioned at an end of an ionization volume 120,the electrons colliding with gas molecules and atoms inside an anodestructure comprising a cylindrical mesh grid that defines the ionizationvolume 120. The method further includes collecting ions formed bycollisions between the electrons and gas molecules and atoms on acollector electrode 110 to provide a gas pressure output. The collectorelectrode 110 extends along a collector axis through the ionizationvolume, the collector axis extending through the ends of the ionizationvolume.

Alternatively, a method of measuring pressure with an ionization gauge200 described above and shown in FIGS. 6A, 6B, and 7 includes emittingelectrons from an electron source 105, the electrons colliding with gasmolecules and atoms inside an anode structure comprising a cylindricalmesh grid that defines an ionization volume 120. The method furtherincludes locating a first shade 115 outside of the ionization volumebetween the electron source 105 and the collector electrode 110, one ofthe collector electrode 110 and the electron source 105 being locatedinside the anode structure 120, and the other of the collector electrode110 and the electron source 105 being located outside the anodestructure 120, and collecting ions formed by collisions between theelectrons and gas molecules and atoms on the collector electrode 110 toprovide a gas pressure output. If the source 105 that emits electrons islocated outside the anode structure 120 and the collector electrode 110is located inside the anode structure 120, then, optionally, the methodcan further include locating a first and a second collector electrode(110 a and 110 b, respectively) inside the anode structure 120 and,additionally, the method can further include locating a third collectorelectrode 210 outside the anode structure 120, in between the firstshade 115 and the anode structure 120, as shown in FIG. 6A. The methodcan further include locating a second shade 119 between the envelope 205and the electron source 105, such that atoms sputtered off the envelope205 are inhibited from depositing on the electron source 105. Thepressure can be in a range of between about 10⁻¹ Torr and about 10⁻⁴Torr.

The relevant teachings of all patents, published applications andreferences cited herein are incorporated by reference in their entirety.

While this invention has been particularly shown and described withreferences to example embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. An ionization gauge to measure pressurecomprising: an extended anode structure comprising a grid extendingalong a longitudinal axis, the grid defining an ionization volume withinthe grid in which electrons impact gas molecules and atoms, oppositeends of the grid defining opposite ends of the ionization volume; a hotcathode electron source that emits electrons into the ionization volume,the electron source being positioned at one of the opposite ends of theionization volume; and an ion collector electrode to collect ions formedby collisions between the electrons, emitted by the hot cathode electronsource, and gas molecules and atoms, to provide a gas pressure output,the ion collector electrode extending through the ionization volumewithin the grid along an ion collector axis substantially parallel tothe longitudinal axis of the grid, the ion collector axis extendingthrough the opposite ends of the ionization volume.
 2. The ionizationgauge of claim 1, wherein the grid is a cylindrical grid.
 3. A method ofmeasuring pressure with an ionization gauge comprising: emittingelectrons from a hot cathode electron source positioned at one ofopposite ends of an ionization volume, the electrons colliding with gasmolecules and atoms inside an extended anode structure comprising acylindrical grid that defines the ionization volume within the grid, thecylindrical grid extending along a longitudinal axis, opposite ends ofthe cylindrical grid defining opposite ends of the ionization volume;and collecting ions formed by collisions between the electrons, emittedby the hot cathode electron source, and gas molecules and atoms on anion collector electrode to provide a gas pressure output, the ioncollector electrode extending through the ionization volume within thegrid along an ion collector axis substantially parallel to thelongitudinal axis of the grid, the ion collector axis extending throughthe opposite ends of the ionization volume.
 4. The method of claim 3,wherein the pressure is in a range of between about 10⁻¹ Torr and about10⁻⁴ Torr.